Plant Hormones

Question 1

What are meristems?

Answer 1

Regions of cells capable of mitotic division and growth in plants

Question 2

What are the two types of growth that can contribute to an increase in the size of a plant?

Answer 2

1. Primary growth:
   - occurs in apical meristem of buds and root tips
   - increases height of plant
2. Secondary growth:
   - increases plant girth and occurs in the lateral meristem in the stem

Question 3

What are three types of primary meristem?

Answer 3

Procambium - vascular tissues
Protoderm - epidermis
Ground meristem - ground tissue

Question 4

What do the three transitional meristems give rise to?

Answer 4

The root’s primary tissue systems - epidermis, vascular tissues, ground tissues

Question 5

Why would plants need hormones?

Answer 5

• controlling growth
• responses to environmental changes
• controlling reproductive cycles
• responses to competition for resources

Question 6

What are two classes of plant hormones? Give examples.

Answer 6

Growth promoters: Auxins, Gibberellins, Cytokinins

Growth inhibitors: Ethylene gas, Abscisic acid

Question 7

How can hormones promote plant growth?

Answer 7

Two ways:
1. Stimulating elongation in cells

2. Stimulating cell division in meristems to produce new cells

Question 8

What are the roles of Auxin?

Answer 8

Tropisms
Apical dominance
Growth of adventitious roots
Fruit growth

Question 9

What is tropism?

Answer 9

Tropisms are the growth of a plant toward or
away from a stimulus, including:
 Phototropism: in response to light
 Gravitropism: in response to gravity
 Thigmotropism: in response to touch

Question 10

What does tropism do?

Answer 10

Tropisms involve
cell elongation or
suppression of cell
elongation on one
side of a plant,
causing the plant
to grow in a
particular direction

Question 11

What is phototropism?

Answer 11

If the tip is exposed to light
on one side, the IAA drifts
to the darker side. This
prompts that side to grow
more (the cells elongate),
bending the tip towards the light source.

Question 12

What is gravitropism (geotropism)?

Answer 12

Auxin concentrates in cells on lower side,
causing differential growth
Roots: cells on lower side are inhibited by auxin, root points downwards towards gracity
Shoots: cells on lower side are stimulated by auxin, shoot points upwards

Question 13

How do cells elongate?

Answer 13

Auxin stimulates proton pumps
pump protons (H+) into cell wall
expansins modify hydrogen bonds between
cellulose molecules
molecules slide past one another, allowing for
elongation

Question 14

What happens when plant cells are illuminated by light from one direction?

Answer 14

Transport proteins in plasma membrane on the shaded side of cell are activated and auxin is transport to the shaded side of plant

Question 15

Why do shoots bend upward?

Answer 15

Auxins are more concentrated on the lower side of stem, causing the cells there to elongate

Question 16

What causes the roots to bend downward?

Answer 16

Auxin concentration on the lower side of the root suppresses cell elongation. The upper side continue to grow and roots bend downward

Question 17

How does a root know which way is down?

Answer 17

Plastids in root cap cell tend to settle on the bottom side of cell and stimulates the release of auxins at the bottom

Question 18

What helps stimulate lateral buds even if the shoot tip is removed?

Answer 18

Cytokinins

Question 19

Why does a plant grows into a tall slender shape?

Answer 19

When the apical bud grows more quickly than the lateral buds

Question 20

What happens if the apical bud is pruned?

Answer 20

The lateral buds are no longer inhibited and the plant grows into a low bushy shape.

Question 21

What are adventitious roots?

Answer 21

Adventitious roots are those growing out of places where roots don’t normally grow. Auxins stimulate root growth on the end of a cutting.

Question 22

What is the correlation between auxins and fruit growth?

Answer 22

• developing seeds produce auxins that stimulate growth of plant ovary into fruit
• removal of seeds from a strawberry prevents the fruit from growing

Question 23

What are gibberellins?

Answer 23

Hormones that cause seedlings to grow excessively tall and t=fall over

Question 24

What are the functions of gibberellins?

Answer 24

 Promotes cell elongation in the internodes of
plants.
 Stimulates growth of the ovary wall into a
fruit.
 Stimulates seed germination and release of
food reserves in seeds.

Question 25

What are the functions of cytokinins?

Answer 25

 Promote growth of lateral buds when auxin
concentrations are low.
 Promote cell division in meristems.
 Stimulate fruit and seed development.
 Delays senescence of plant parts. (senescence: loss of cells power to grow and divide)

Question 26

What are the functions of ethylene?

Answer 26

 Released by fruits and causes the fruits to
ripen faster.
 Causes plant parts to age and die
(senescence).
 Inhibits stem elongation.

Question 27

What are the functions of abscisic acid?

Answer 27

 Controls seed and bud dormancy.
 Inhibits gibberellins.
 Promotes senescence in plants.

Question 28

Why do flowers grow?

Answer 28

Changes in gene expression in the shoot apex causes the meristem to produce flowers instead of leaves

Question 29

What are the roles of auxins and cytokinins in the roots?

Answer 29

Auxins: gravitropism in roots
Cytokinins: Ensure roots and shoots grow at equal rates

Question 30

How does abscisic affect water loss?

Answer 30

It controls the closing of stomata and hence regulates the water loss

Question 31

What do abscisic acid and gibberellins do for a seed?

Answer 31

Acid: seed dormancy
G: triggers germination in dormant seeds

Question 32

What does auxins and gibberellins do at the shoot?

Answer 32

Auxins: promote apical dominance and primary cell growth
G: causes stem elongation by promoting cell elongation and cell division

Question 33

What is the role of cytokinins, auxins, and ethylene in fruit growth?

Answer 33

Cytokinins: stimulating fruit growth

Auxins: promote fruit growth but inhibit ripening

Ethylene: ripening of certain fruit

Question 34

What is the role of ethylene and abscisic acid in leaves?

Answer 34

E: loss of leaves and death of flowers
Acid: initiates stress responses in plants